The pH measures the activity of hydrogen ions H+ in solution, especially in aqueous solution where these ions form oxonium ions H3O+ (also called hydronium) with water. The pH reflects the acidity of the solution. For example, in an aqueous medium at 25° C., a pH of 7 is called neutral, a pH above 7 is called basic and a pH below 7 is called acidic.
Since acidity is a parameter to consider in metal corrosion mechanisms, and especially in metal ducts for industrial plants, knowing the pH enables to control corrosion of these ducts.
Acidity is also a parameter to consider in the solubility of chemical species in aqueous solution. Therefore, knowing the pH enables to control deposition of chemical compounds on surfaces due to crystallization of these compounds at certain pHs. Thus, it is possible, for example, to prevent fouling of surfaces and clogging of steam generators.
Finally, certain industrial procedures require operation in a limited pH range and therefore require measuring the pH.
The most widely used pH measuring devices are based on a glass membrane. These measuring devices only work at temperatures below 100° C.
Other measuring devices can work up to 280° C. These devices couple a reference Ag/AgCl electrode with a polymer electrode body, on the one hand, with a platinum electrode immersed in the solution whose pH is to be measured, on the other hand. The thermal stability of the AgCl salt and the melting temperatures of the polymers used to fabricate the electrode body require a limit of operative temperature of 280° C. Furthermore, the possible presence of chloride can damage iron-based metal circuits and that of silver is extremely harmful for nuclear circuits.
There are devices for measuring above 280° C. For example, yttrium-zirconium oxide membrane devices work above 300° C. However, they do not enable to go below 300° C. because diffusion of oxygen into the oxide decreases exponentially with temperature. The diffusion of oxygen is insufficient to make reliable measurements below around 300° C. These measuring devices also do not measure the activity of the hydronium ions of the solution, but rather the partial pressure of oxygen.